1. Field of the Invention
The invention relates to a method for operating a binaural hearing apparatus with frequency distortion and a binaural hearing apparatus with frequency distortion.
In hearing apparatuses, in particular in hearing devices, frequency-distorting algorithms are used for different purposes and at different points in a signal processing. German utility model DE 699 22 940 T2 discloses a hearing device with a combination of audio compression and feedback suppression for instance.
The frequency-distorting algorithms unfortunately also produce clearly perceivable artifacts. A distortion at low frequencies is generally not possible since the human ear reacts very sensitively in the low frequency range. Only the high frequencies are therefore distorted. Nevertheless, this may result in an audible “detuning” of a useful signal.
Superimposition artifacts are considerably more unpleasant, in which the distorted signal and the undistorted signal are perceived at the same time, thereby resulting, in the case of tonal signals, in a clear modulation and/or beat frequency or a roughness. Acoustic superimpositions, which take place as a result of the inflow of direct sound through the vent for instance, are almost completely unavoidable. Superimpositions as a result of non-ideal split-band filters may also result however as a result of the design type. To be able to only distort high frequency parts, these must be separated from the low frequency parts. To this end, a split-band filter is needed. The split-band filter can however not carry out an ideal separation, as a result of which interfering superimpositions result in the region of the cut-off frequency.
As a function of the frequency distortion, these superimpositions are perceived as amplitude modulation or as signal roughness. In all described cases, the superimpositions are interfering, particularly if an input signal is music or more generally tonal signals.
FIG. 1 shows a block diagram of an exemplary realization of a frequency distortion in a hearing device. An input signal 100 is divided by a split band filter 1 with a predeterminable cut-off frequency GF (split frequency) into a low frequency and a high frequency signal part 101, 102. The high frequency signal part 102 is then distorted in a frequency distorter 2. The distorted output signal 103 is fed to an input of an adder 3. The low frequency signal part 101 passes through an all-pass filter 4, which rotates the phase of the signal part 101 such that in the case of a subsequent signal addition in the adder 3, signal deletions do not result in the region of the cut-off frequency GF. The phase-rotated low frequency signal part 104 is fed to a further input of the adder 3. The total of the two signal parts 103, 104 is available as an output signal 105 at the output of the adder 3.
Split-band filters are not ideal and have an endless frequency overlapping in the case of their cut-off frequency GF. FIG. 2 shows an example of the frequency response of a split-band filter in a hearing device with the cut-off frequency GF of 1800 Hz. The curves K1, K2 indicate the attenuation D in dB as a function of the frequency F in Hz in the range between 0 to 4000 Hz. The curve K1 shows a low-pass characteristic and the curve K2 shows a high-pass characteristic.
If a low-pass K1 filtered signal part is now not distorted and a high pass K2 filtered signal part is distorted, when the signal parts K1, K2 are added, this results above all in the region of the cut-off frequency GF in a not insignificant superimposition of both signal parts, which, in an output signal of the hearing device, is perceived as modulation or significant roughness. Both effects are very interfering and, in terms of the perception of a hearing device wearer, are in most cases significantly more obvious than the frequency distortion itself.
Strong frequency-distorting algorithms are generally used in the case of significant hearing losses, with artifacts being accepted and/or not perceived by hearing-impaired persons. Problems nevertheless also cause weak frequency-distorting algorithms, which are used for instance to assist with feedback suppression. Since these are to be useable for all hearing device wearers, they must be as inconspicuous as possible. An on/off logic is therefore currently used above all, which activates the frequency distortion when feedback artifacts are surmised and which switches off the frequency distortion when no feedback is surmised. This logic is in this case certainly disadvantageous in that a feedback whistling first has to be detected before the algorithm is switched on, which then in turn requires a certain amount of time until it achieves its full effect. This delays the feedback suppression and runs the risk of fault recognition.
Published, European patent application EP 1 333 700 A2 discloses a method and a hearing device for frequency shift purposes. A shifted spectrum is obtained here from the spectrum of a microphone signal of the hearing device by a non-linear frequency shift function.